1. Field of the Invention
The present invention relates to a device for reactivating a catalyst of an engine.
2. Description of the Related Art
Typically, a catalyst arranged in an exhaust passage of an engine is provided with a porous carrier which has an adsorbing function. However, the exhaust gas discharged from the engine includes a soluble organic fraction (SOF) and the SOF is adsorbed in the porous carrier. However, if the SOF is adsorbed in the inner portion of the porous carrier which is spaced apart from the surface of the carrier, it is difficult for the SOF to be desorbed therefrom. Also, a porous carrier such as zeolite has a catalytic function by itself and, therefore, the SOF may be reformed in the porous carrier if the SOF stays in the porous carrier for a long time. If the reformed SOF covers the catalytic metal or blocks the micro pores of the porous carrier, NO.sub.X and the reducing agent cannot reach the catalytic metal and, therefore, the NO.sub.X can not be reduced sufficiently. In this way, the catalyst is poisoned by SOF.
However, the SOF will be burned when the temperature of the porous carrier becomes high in an oxidizing atmosphere. Therefore, Japanese Unexamined Patent Publication No. 6-200737 discloses a device for reactivating a catalyst of a diesel engine in which it is judged whether the catalyst is poisoned by SOF and, if the catalyst is judged to be poisoned, the catalyst is heated so that the SOF in the catalyst is burned, to thereby reactivate the catalyst.
In this device, the SOF existing on or near the surface of the porous carrier is relatively easily heated and thus burned. However, it is difficult to heat the SOF existing in the inner portion of the porous carrier to a temperature where the SOF can be burned. Namely, it is difficult for the catalyst to be reactivated sufficiently.